Critical loads: concept and applications

The concept of a 'critical load' is coming to dominate European legislation on air pollution. A critical load can be defined as 'the maximum deposition of a given compound which will not cause long-term harmful effects on ecosystem structure and function, according to present knowledge'. It is intended, in other words, to be a threshold deposition which ecosystems can tolerate without damage. One aim of the pollution control policies of many countries is now to reduce the deposition of acidifying pollutants below their critical loads. To enable this to be possible, the scientific community has to be able to define well-supported critical loads for any European ecosystem. As an attempt to codify a vast amount of ecological knowledge and use it as a rational means of pollution control, the critical loads approach is an ambitious undertaking. Maps of critical loads for various receptors have now been published for the whole of Europe by several bodies on a variety of scales. In the UK, the Department of the Environment has published maps of critical loads on scales down to 1 km. The concept is being used by the UK government as a guide for its policies on pollution control, and by European governments under the auspices of the United Nations Economic Commission for Europe as a basis for negotiations for a new protocol on limiting long-range trans-boundary sulphur pollution. The concept, clearly, is of great political importance. The development of the concept and production of the maps have taken place with remarkable speed, generally within working parties whose composition has been determined by the governments of the countries concerned. There has been little opportunity for the ecological community as a whole to participate in these discussions, or for those who have been involved to present the results of their deliberations to their scientific peers. To remedy this situation somewhat, we organised a Conference open to all interested parties, and invited contributions from both scientists and those whose job it is to develop and implement environmental policies. The Conference was held at Grange-over-Sands on 12-14 February 1992, under the auspices of the British Ecological Society Industrial Ecology Group and the Natural Environment Research Council (NERC). The National Power/PowerGen Joint Environmental Programme provided some sponsorship. The Conference generated some new ideas on critical loads, showed that there was a variety of views on the validity of the concept as it stood, and sparked off some controversies. This volume documents the proceedings of the Conference, both written papers provided by the speakers and reports of workshop discussions on specific questions. We hope it will be of use in further development of the critical load concept, and of interest to anyone who wishes to understand the progress made so far.